Is Oxygen Dangerous?

I was sitting at a table of all Thai colleagues and discussing walking through the park next to my house.  I was told and it was unanimously supported by the table that walking in the midst of all the carbon dioxide that the trees put out is dangerous and I should not do it so much.

I took a second and put on my "are you smarter than a 5th grader" hat and said trees emit oxygen not CO2.  Further more, it isn't ever dangerous to be near a tree for the O2 it is putting out, lightening yes, O2, NO.  This caused a table discussion where everyone, not joking and dead serious, said that is what they all learned in school.

What is the country teaching its youth?